a. The Field of the Invention
This invention is concerned with forming and using light retroreflective systems particularly adapted for being utilized in highway marking signs and advertising signs, to provide a distinguishing surface, and capable of reflecting light from such sources as headlight beams, for ensuring visibility at a distance in nighttime. The term retroflection is hereinafter used to designate a reflective means, element or system which will reflect an incident beam or ray of light in such a manner that a brilliant beam, generally a cone of light, is returned to the source of light even though the incident light strikes the surface of the retroreflective system at an angle.
B. The prior Art
This art is well known and extensive comments thereabout are believed to be unnecessary. Usually, the retroreflective system comprises retroreflective elements adapted to be partially embedded within a layer of a suitable composition, such as at a highway lane dividing line or otherwise in a traffic regulating sign formed on the road pavement, or in a preformed tape material designed to be applied on and adhesively secured to said pavement.
Some of such retroreflective systems comprise small optical systems each of which consists of a primary globular (generally spherical) transparent body which, when partially embedded in the sign forming material, has a part-spherical surface which projects exteriorly of the surface of the sign, the primary body having a refractive index which causes a light beam impinging on said exposed part-spherical surface to refract within said body and focus at a location of the surface of the body, which is approximately opposite to the area of light impingement, located below the sign surface and adjacent to a reflective arrangement, such as reflectorized microspheres, so that the impinging beam is reflected back along a path essentially parallel to its own incident path. These principles and phenomena are discussed in detail. For instance, in the specification of the British Pat. No. 1,343,196.
According to well known optical laws, a proper focusing as above, in a spherical lens, of a light beam impinging on a satisfactorily large area of the part-spherical exposed surface of the primary body, occurs only when such body is formed of a transparent material having a refractive index of from about 1.90 to 1.95. Therefore, to provide an optically efficient retroreflecting system of the prior art, costly, brittle and poorly weather resistant high refraction glasses are to be used. Additionally, the use of high refraction transparent material leads to an undesirable light dissipation by outward reflection at the primary body-air interface when the light ray impinges on the exposed surface of the body at a relevant angle of incidence.
In the U.S. Pat. No. 3,781,083, there is described an arrangement including spherical primary transparent bodies which can be made of a material having an index of refraction well below 1.90, such as from 1.45 to 1.68. This advantageous feature is made possible by the fact that the reflecting arrangement, such as reflectorized microspheres, is located at distance from the primary body, the light beams impinging on and refracted within said bodies focusing outside of said bodies. This arrangement cannot be successfully utilized for forming structurally unitary small retroreflecting systems.
Therefore, this invention has as its objects the provision of a novel and improved light retroreflecting system of the above type, the provision of a new and advantageous solution to the above and other problems, and more specifically the provision of a new optically improved retroreflecting system including a transparent primary body of a material having an index of refraction substantially lower than 1.90, such primary body being so structurally combined with a reflectorized reflecting arrangement that the most desirable high retroreflective efficiency can be achieved together with other advantages relating to traffic resistance and service time.
Essentially, according to the invention, there is provided a retroreflecting system including a primary focusing transparent body adapted to be partially embedded into a layer of a sign forming material and to provide a convex surface exposed to light impingement above the sign surface, said convex surface including portions adjacent to said sign surface of radius of curvature less than half of the path along which a light ray impinging on a location in said portion travels within said primary body towards a reflective means adjacent to said body and located below said sign surface.
Inasmuch as in a convex lens the distance from the convex surface impinged by a light beam and the point at which such beam is focused by the lens is inversely porportionate to the radius of curvature of such surface and to the index of refraction of the transparent material of which the lens is made, the geometry of the primary body of the new system is such as to provide the most proper focalization, and consequent correct retroreflection, of a beam of "grazing light," that is the light beams of interest which impinge on the exposed portion of the primary body in a direction forming an extremely small angle (such as 1.degree. or 2.degree.) with the sign surface, at a location nearly opposite to the area of impingement and below the sign surface, where the reflective means are adjacent, by forming such a primary body of a material having an index of refraction well below that necessary for providing a proper focalization in a spherical lens of the same radius of curvature.
Preferably, the geometry is such that the contours of the cross-sections of the primary body, in planes perpendicular to the sign surface, are elliptical at least in the exposed portions above said sign surface.
A retroreflective system according to the invention can therefore comprise a primary body consisting of the well known, economical and weather resistant "window pane glass" of index of refraction n.sub.D = 1.50-1.55. Most preferably, such primary body is formed of an advantageously less brittle, more weather resistant and only progressively worn off transparent polymeric composition, such as polyester resins (n.sub.D = 1.52-1.56), polystyrene resins (n.sub.D = 1.58-1.60), polyurethane resins (n.sub.D = 1.47 to 1.54), polycarbonates (n.sub.D = 1.58-1.60), and acrylic and metacrylic polymers (n.sub.D = 1.46-1.63), and their chemically compatible copolymers and mixtures.
These and other objects, advantages and features of the invention will become apparent from the following detailed description of some not limitative embodiments thereof, taken together with the accompanying drawings.